Most structural steels used as components of mechanical structures and vehicles are quenched and tempered steels obtained by increasing strength and toughness through reheating, quenching, and tempering after hot working. On the contrary, a non-heat treated steel is a steel that does not undergo a heat treatment after hot working, but has a similar toughness and strength to a steel that has undergone a heat treatment (heat treated steel). Non-heat treated steel may be also called a “micro-alloyed steel” since the quality of the material is obtained by adding a very small amount of an alloying element.
Generally, a typical wire rod product is produced as a final product through the operations of Hot Rolling→Cold Drawing→Spheroidization Heat treatment→Cold Drawing→Cold Forging→Quenching and Tempering, whereas a non-heat treated steel is produced through the operations of Hot Rolling→Cold Drawing→Cold Forging→Product.
As described above, the non-heat treated steel is an economical product that may be produced without heat treatment and at the same time, does not undergo a final quenching and tempering process. Therefore, non-heat treated steel has been applied to many products due to the securing of linearity obtained by not generating heating deflections, i.e., defects caused during heating.
However, since the heat treatment process is omitted and cold working is continuously applied, as the processes progress, the strength of the product is further increased, while ductility is continuously decreased. To solve such drawbacks, the following techniques are disclosed.
Japanese Patent Laid-Open Publication No. 1995-054040 discloses a method for providing a non-heat treated steel wire rod with 750-950 MPa of tension by hot rolling the alloy steel that is composed of C: 0.1-0.2%, Si: 0.05-0.5%, Mn: 1.0-2.0%, Cr: 0.05-0.3%, Mo: 0.1% or less, V: 0.05-0.2%, Nb: 0.005-0.03%, and the remainder Fe, as a percentage by weight, cooling the alloy steel within 60 seconds between 800-600° C. for a cooling operation, and heating at 450-600° C., or cooling the alloyed steel after continuously maintaining it at a temperature of between 600-450° C. for at least 20 minutes, and then cold working. However, the product is hot-rolled through a process known as controlled rolling, and relatively expensive components such as chromium (Cr), molybdenum (Mo), vanadium (V), and the like are added in the method as mentioned above, so that it is uneconomical in practical use.
In addition, Japanese Patent Laid-Open Publication No. 1998-008209 relates to non-heat treated steel with excellent strength after hot working, and excellent cold formability and a method for manufacturing the same, and a method for preparing a forging member by using a non-heat treated steel, and also relates to non-heat treated steel with excellent cold formability, in which a volume of a ferrite phase is at least 40%, and a hardness is 90 HRB or less, for the steel having a controlled contents of carbon (C), silicon (Si), manganese (Mn), Cr, V, phosphorus (P), oxygen (O), sulfur (S), tellurium (Te), lead (Pb), bismuth (Bi), and calcium (Ca). Specifically, the document relates to a method for continuously cooling to an Al point temperature or less at cooling rate of 120° C. or less per minute, immediately after hot-rolling at 800-950° C. during a final working temperature, a method for cooling a hot rolled steel material in the air after heating for at least 10 minutes at 800-950° C., and also, a method for preparing a structural member with 20-35 HRB of hardness by cold working or warm working at a temperature of 600° C. or less, preparing a preform, and air cooling after hot-forging the preform at 1000-1250° C. However, the technology is limited to a specific steel containing elements that are usually not used, and is not applied to cold forging.
In addition, Japanese Patent Laid-Open Publication No. 2006-118014 provides a method for manufacturing case-hardened steel that is suitable for a bolt, and the like, which suppresses grain coarsening after heat treatment, even if cold formability is excellent and also, working with a high cutting rate of an expanded line is performed. The method as mentioned above uses the steel material that is composed of C: 0.1-0.25%, Si: 0.5% or less, Mn: 0.3-1.0%, P: 0.03% or less, S: 0.03% or less, Cr: 0.3-1.5%, aluminum (Al): 0.02-0.1%, N: 0.005-0.02%, the remainder iron (Fe), and other inevitable impurities, as a percentage by weight, and the method for manufacturing non-heat treated wire rod with excellent toughness is achieved by performing hot finish rolling or hot finish forging at 700-850° C., then cooling by up to 600° C. at a cooling rate of 0.5° C./sec or less, and suppressing a cut rate of an expanded line to below 20% by cooling to room temperature. The technology as mentioned above discloses the use of a small amount of Mn, and the use of Cr and Al.